Fix URL for download of sample test cases

[avborup]

It seems that the download URL for the sample test cases zip file depends on the host URL, not the particular problem URL.

I.e., if you get the problem at the URL https://itu.kattis.com/courses/KSALDES1KU/KSALDES1KU-2023/assignments/nute7h/problems/pebblesolitaire2, the zip file with test cases is not bound to that course URL, but rather to the "simple" problem URL: https://itu.kattis.com/problems/pebblesolitaire2/file/statement/samples.zip.